Morphological studies on the epiphyseal growth plate combined with biochemical and X-ray microprobe analyses

Summary The epiphyseal growth plate of the domestic pig was investigated topologically combining biochemical methods with electron microprobe microanalyses both correlated to histological controls. A lateral resolution of about 50 m was reached. Highest nuclease activity was found in the lower columnar cell zone, while alkaline phosphatase showed maximal activity in the hypertrophic area, connected with maximal values for extractable, organically bound phosphorus, and extractable Ca and Mg. Acid phosphatase activity reached maximal values in the zone of the lower primary spongiosa, while the extractable P_i had maximal values at the end of the zone of bone remodelling. Microprobe analyses have shown that the extracellular Ca content (per dry mass) remained relatively constant at 0.7% (about 58 mM/kg wet weight for 66% tissue fluid) in all zones of the plate increasing to 1% in the vicinity of the first foci of mineralization. The intracellular P content (per dry mass) was about 4.5%, the extracellular 0.1–0.2% (about 10–20 mM/kg wet weight) increasing also to about 1% in the vicinity of the first foci of mineralization. Thus the Ca x P product was much higher than the ion-product of 2 mM² which is necessary for an in vitro mineralization of connective tissue. The extracellular S content (per dry mass) as a probable indicator of sulfated proteoglycans was relatively constant at about 3.5% in the different zones but decreased to about 0.3% in the fully mineralized regions. This indicates a loss of sulfur containing substances with mineralization which is not so high since the concentrations per dry mass must be normalized to a unit volume of equal density of mass.